S e s s i o n 3: Fi sh b e h a v i o u r a n d th e q uality o f a c o u s t i c d a ta e f f e c t s o f s a m p l i n g , v e s s e l s ,
and survey design
Rapp. P.-v. Réun. Cons. int. Explor. Mer, 189: 112-122. 1990
Fish behaviour: achievements and potential of high-resolution
sector-scanning sonar
G . P. A r n o l d , M . G r e e r W a l k e r , an d B . H . H o l f o r d
Arnold, G. R , G reer Walker, M.. and Holford, B. H. 1990. Fish behaviour: achieve­
ments and potential of high-resolution sector-scanning sonar. - Rapp. P.-v. Réun.
Cons. int. Explor. Mer, 189: 112-122.
High-resolution sector-scanning sonar, operating at a frequency of 300 kHz. has been
used in conjunction with a series of transponding acoustic tags to observe the
behaviour of unrestricted, identified, individual fish in the open sea. Observations of
the behaviour of individual fish have been used to predict patterns of behaviour in
natural populations, and these predictions have been tested by independent methods.
The advantages and disadvantages of existing techniques of acoustic telemetry are
discussed in the context of future research requirements and possible technical
developments.
G. P. Arnold, M. Greer Walker, and B. H. Holford: Ministry o f Agriculture, Fisheries
and Food, Directorate o f Fisheries Research, Fisheries Laboratory, Lowestoft, Suffolk
N R 330H T , England.
Introduction
The Fisheries Laboratory, Lowestoft, has been associ­
ated with the application of sector-scanning sonar to
fisheries problems for 25 years (Harden Jones and
McCartney, 1962; Cushing and Harden Jones, 1966)
and has been actively engaged in acoustic telemetry
since 1970, following the installation in RV “CHone”
(Mitson and Cook, 1971) of the high-resolution sectorscanning sonar invented by Dr G .M . Voglis of the
Admiralty Research Laboratory (A R L ), Teddington.
The A R L scanner and its solid-state counterpart (the
MA FF scanner) developed at Lowestoft (Holley et al. ,
1975) have been used to study some of the aspects of
fish behaviour that are thought to determine the distri­
bution of fish populations or to bias indices of pop­
ulation abundance derived from trawling or acoustic
surveys.
The scanners have been used in conjunction with a
transponding acoustic tag, which returns a powerful and
unambiguous signal when ensonified by the outgoing
signal of the sonar. The tag is small (5 cm long x 1 cm
in diameter) and unlikely to affect the swimming per­
formance of the fish in any significant way (Arnold and
Holford, 1978). Various technical improvements have
© British Crown copyright, 1990.
112
been made to the tag since its original invention (Mitson
and Storeton West, 1971), and two telemetric tags have
been developed from it. One telemeters heart rate in
real time (Storeton West et a i , 1978); the other - a
much larger tag - measures the compass heading of the
fish assigned to one or other of eight nominally 45°
sectors (Mitson et al., 1982; Pearson and Storeton West,
1987). When the compass tag is interrogated at 300 kHz
it responds with a reference pulse, which is used to
determine the position and depth of the fish in the usual
way (Greer Walker et al. , 1978). The compass heading
is indicated by a second pulse and the delay between the
two signals (ranging from 24 to 66 ms in seven 6-ms
steps) identifies the relevant sector. The two pulses are
shown on the B-scan sonar display and on a paper
recorder (Harden Jones, 1981; Harden Jones and
Arnold, 1982). The M A FF scanner with a scan con­
verter, improved controls, displays and new data-logging facilities has been installed in our new research
vessel RV ’‘Corystes" which was commissioned in 1988
and which has replaced RV “Clione” .
The key features of the acoustic tracking system are:
(i)
an imaging sonar, which can reveal details of fish­
ing gear, wrecks, and bottom topography;
(ii) the ability to determine the position of individually
identified targets in three dimensions (depth is very
important);
(iii) the ability to make observations independently of
underwater visibility and ambient light intensity;
(iv) a mobility which matches that of free-ranging fish;
and
(v) the ability to telemeter environmental or physio­
logical data from the tagged fish back to the ship.
The technique has been applied to three main areas of
research: (1) to describe the reactions of plaice (P le u ro nectes p la te ss a ) to a Granton otter trawl and measure
the efficiency of the gear; (2) to follow the movements
of migratory fish in the southern North Sea in relation
to the tidal streams; and (3) to telemeter the compass
heading from free-ranging fish.
The key contribution of the technique has been that it
has allowed us to observe the behaviour of unrestricted,
identified, individual fish in the open sea. From these
observations, we have been able to predict patterns of
behaviour in natural populations and test these pre­
dictions by independent methods. This point is illus­
trated by reference to two examples: migration by selec­
tive tidal stream transport; and vertical migration of cod
in relation to pressure. The estimates of the efficiency of
the Granton trawl have already been published (Harden
Jones et a i , 1977; Harden Jones, 1980; Harden Jones
and Arnold, 1982), and the descriptions of the reactions
of individual fish to the various components of the gear
- the subject of a separate evening presentation to the
International Symposium on Fisheries Acoustics in
Seattle in June 1987 - will be published elsewhere.
M i g r a ti o n by s e l e c t i v e tidal s tr e a m
tr an sport
Water currents can provide transport or directional in­
formation for migrating fish. On the European conti­
nental shelf, where tides are an important feature of the
physical environment, we believe that stocks of de­
mersal fish may be largely contained within the bounda­
ries of the tidal stream paths. O ur belief is based on the
demonstration of a pattern of vertical migration linked
to the tidal streams, which we have called selective tidal
stre a m tra n sp o rt (Greer Walker et a l., 1978), and which
we have observed in species as diverse as plaice, sole
(S o lea so le a ), cod ( G a d u s m o r h u a ), dogfish ( S c y lio r h in u s c a n ic u la ), and silver eels (Anguilla a n g u illa ).
Selective tidal stream transport
The transport system is deceptively simple. The fish
leaves the bottom at one slackwater and is carried
downstream for part or all of the duration of the trans­
porting tide. It returns to the bottom at the next slack­
water and remains there for the duration of the oppos­
ing tide, making no significant movement. The fish
8
R apports et Procès-Verbaux
moves up into midwater approximately every 12 h and
the vertical migration has a semi-diurnal periodicity.1 In
plaice, such behaviour is characteristic of the seasonal
spawning migrations and thus of large-scale geograph­
ical movements. On the feeding grounds in summer and
the spawning grounds in winter, there is a different
pattern of behaviour. The vertical migration then has a
diurnal (24 h) periodicity and a proportion of the pop­
ulation moves into midwater at night. These vertical
movements are not linked to a particular direction of
the tidal stream and appear to be associated with local
movements in search of food or a spawning partner. At
both scales, the net direction of movement over the
ground is determined primarily by the tidal stream, and
transport appears to be essentially passive. We think the
fish may obtain information about the speed and direc­
tion of the tidal stream when it is on, or close to, the
seabed but not when it is in midwater.
Evidence
Evidence in support of these hypotheses comes from:
(1) tracks of individual acoustically tagged fish (Greer
Walker et a l., 1978, 1980; Arnold, 1981; Harden Jones
and Arnold, 1982; Arnold and Cook, 1984; McCleave
and Kleckner, 1985; and additional unpublished obser­
vations); (2) midwater trawling experiments for plaice
on feeding grounds (Arnold, 1981) and spawning
grounds (Arnold and Cook, 1984) in the southern North
Sea and along the line of the western migration route
into the Southern Bight (Harden Jones et cd., 1979);
and (3) the results (still largely unpublished) of experi­
ments with plaice fitted with transponding acoustic com­
pass tags (Harden Jones, 1981, 1984; H arden Jones and
Arnold", 1982).
C om pass heading
Seven plaice fitted with transponding acoustic compass
tags were tracked in the southern North Sea in 1979 and
1980. Although we have not yet completed the analysis
of all the data collected during these tracks, it is already
clear that the orientation of plaice on, or close to, the
seabed is influenced both by the speed and direction of
the tidal stream and by local bottom topography. And
in midwater, although they may at times go round in
circles (one fish completed nine consecutive circles in 80
minutes), plaice can maintain a surprisingly consistent
heading, remaining within two 45° compass sectors for
an hour or more (Harden Jones, 1981, 1984). At the
same time, their heading in midwater is apparently un'The terms diurnal and semi-diurnal are used here in the same
context as in physical oceanography (Defant, 1958, page 48),
to describe tides and other cyclical phenomena with periods of
about 24 and 12 h respectively, and not with the alternative
connotation of day versus night.
113
+100
E o
o 2 +50
n*644
n = 1102
~o
<D
CD
Q.
tn
E
co
a>
cc
■o
5
-50
-1 0 0
OB
MW
0600
0200
1400
GMT
1000
1800
22 00
24 May 1980
Figure 1. The compass heading of plaice 3 in relation to the speed and direction of the tide while the fish was on the bottom (OB)
on 24 May 1980. The compass heading was sampled every 20 s. The time recorded in each sector is expressed as a proportion of
the total num ber of observations for each of three periods (0318 -0 9 2 5 h, 0925-1600 h, 1645-2104 h) indicated by vertical
dashed lines. No data were recorded between 1600 and 1645 h. The length of the radius between the inner and outer circles
corresponds to 100 % and + indicates observations amounting to less than 2 % of the total. The predicted directions of the northand south-going tidal streams were 15° and 196°, respectively. The fish was in midwater (MW) from 2104 h.
affected by a reversal in the direction of the tidal
stream. These points are illustrated here by reference to
previously unpublished data from selected portions of
plaice compass tracks 2, 3, and 4.
For example. Figure 1 shows that plaice 3 reversed its
heading twice to oppose the direction of the prevailing
Excluding
touchdowns
Mean
current
50cm s _1
tidal stream during an extended period on the bottom.
On each occasion the change of heading occurred 1 - 2
hours after slackwater, during the accelerating phase of
the tide. Plaice 2 headed downstream (compass sectors
6, 7, and 8) during the course of four successive ex­
cursions to the seabed (Fig. 2). These visits to the bot-
Touchdowns
6
E
o
o 20
n = 26 0
n = 40
n
a>
>
0
A
cd
n
D>
<D
1
0 L l_
220 0
2230
GMT
114
2300
4 June 1979
2330
Figure 2. The compass heading of
plaice 2 during an extended period
in midwater on 4 June 1979. The
changes of heading apparently
associated with the brief periods of
touchdown occurred during the time
intervals indicated by the horizontal
black bars. The compass sectors are
identified by the numbers 1 to 8 in
the right-hand circle.
0900
a
! 1
Figure 3. The compass heading
of plaice 4 in midwater for
eleven consecutive 15-min
periods between 0900 and 1145 h
on 27 May 1980. The compass
heading was sampled every 20 s,
and each circular distribution is
therefore based on a total of 45
samples. The geographic position
of the fish is indicated by open
and closed circles, which
correspond to north-going and
south-going tides, respectively.
The half-closed circle indicates
the predicted time of reversal of
the tidal stream at local highwater.
P re ssure an d vertical m i g r a t i o n o f c o d
tom occurred at intervals throughout a period of down­
stream transport in midwater, during which the fish
otherwise maintained a cross-tide heading in compass
sectors 1 and 2. At this time of night (sunset 2000 h,
sunrise 0330 h, approximately) the fish is unlikely to
have had a visual reference point when it went to the
bottom, and may therefore have oriented to the current
as a result of tactile stimuli. Plaice 4 (Fig. 3) experienced
three reversals of tide during a 20-h period in midwater
at the end of a 48-h track, and remained entirely in
midwater for two of them. On each occasion, the fish
adopted a heading at the end of a south-going tide and
maintained it for a period of 2 to 2.5 h, extending over
the ensuing high-water slack and into the initial period
of the next north-going tide. During the first tidal
stream reversal, plaice 4 maintained an easterly heading
at night for a period of 2.25 h; during the second re­
versal, it maintained a southwesterly heading by day for
2.5 h (Fig. 3). On each occasion, the heading subse­
quently changed only after the fish had made an ex­
cursion to the seabed. As a plaice in midwater at night
has no visual reference point against which it can detect
its displacement by the current, it seems that some other
reference system must account for the consistency of
heading in midwater. Inertial or magnetic clues are
obvious possibilities.
8-
The swimbladder is the main source of acoustic reflec­
tion in those fish that have one. Changes in target
strength that accompany vertical migration as a result of
a change in body attitude, or a change in swimbladder
volume, are therefore of interest as a source of bias
when acoustic methods are used to estimate fish abun­
dance. Some of our observations on the behaviour of
acoustically tagged cod in the southern North Sea are
relevant to this problem.
Physiology and behaviour
The cod has a closed compliant swimbladder which
occupies 5 % of the volume of the fish. Pressure changes
caused by vertical movements lead to expansion or con­
traction of the swimbladder gas and the fish responds to
the accompanying changes in density with compensa­
tory swimming movements and resorption or secretion
of gas. Secretion is rather slow and is temperature de­
pendent. Resorption, which is very much faster, is pres­
sure dependent, and the difference between the two
rates increases with pressure (Harden Jones and
Scholes. 1985).
The rates of secretion and resorption impose limita­
tions on the extent of any vertical movements that the
fish may make while remaining neutrally buoyant. U n ­
der experimental conditions, cod can cope with reduc115
tions of 25 % and increases of 50 % of the pressure to
which they were originally adapted without showing any
abnormal behaviour (Harden Jones and Scholes, 1985).
These limits define a free vertical range within which
the fish should be able to move without experiencing
any exceptional buoyancy problem. The range of verti­
cal movement is substantial; it increases with depth and
the fish has greater freedom of movement in the down­
ward direction.
Classically, the swimbladder has been regarded as a
hydrostatic organ which enables the fish to remain in
neutral buoyancy at all times of the day and night,
regardless of depth. More recently it has been argued
that fish which make extensive vertical migrations are in
neutral buoyancy only at the top of their vertical range
(Konstantinov, 1965; Alexander, 1966; Tytler and Blaxter. 1973; Blaxter and Tytler, 1978). And Harden Jones
and Scholes (1985) point out that the results of their
laboratory experiments could be reconciled with field
observations if cod, which remained on or near the
seabed by day, had an incompletely filled swimbladder
and were negatively buoyant when on the bottom. D ur­
ing vertical migration, these fish would be expected to
swim up into midwater until they reached (he depth at
which they were in neutral buoyancy following the nat­
ural expansion of the swimbladder with reduced pres­
sure. The swimbladder would thus set an upper level to
the extent of the vertical migration, but the rate of
vertical movement and the time spent in midwater
would both be free from restriction. Our observations
are generally consistent with this second hypothesis,
and the rates of descent of fish in midwater are in
agreement with the rates of secretion determined exper­
imentally by Harden Jones and Scholes (1985).
Tracking experim ents
We have tracked 24 cod in the southern North Sea
(Arnold et al., subm.). Three fish were released at the
surface, the others from cages on the seabed at depths
of 24 to 73 m. Each fish was allowed an adaptation
period judged to be appropriate to the particular depth;
the longest period was 150 h.
We have assumed that, on release, each fish adopted
a depth in the water column close to that at which it
would have been neutrally buoyant had it been secret­
ing gas at the expected rate for the whole adaptation
period. We have calculated the expected depth of neu­
tral buoyancy for each fish from the predicted rate of
descent, the adaptation time, the sea temperature, and
the depth of the cage, and compared it with the ob­
served depth of the fish on release.
Nine of the 21 fish released at the seabed appeared to
have achieved neutral buoyancy, in that they remained
within the upper limit of the free vertical range for the
predicted depth of neutral buoyancy. The other 12 fish
took up a depth in midwater which was shallower than
116
Tabic 1. Observed and predicted rates of adaptation of cod in
midwater (Arnold el al., subm.).
Fish
Depth
of
cage
(m)
Observed
rate of
adaptation
in midwater
(m h _I)
Predicted
rate of
adaptation
Ratio:
observed rate
predicted rate
(m h ')
(a) Cod released at the surface
2
3
0.83
0.46
0.84
0.83
1.0
0.6
0.74
0.72
0.72
0.74
0.4
1.0
1.0
0.4
(b) Cod released al the seabed
9
10
11
14
23
40
44
36
0.3
0.7
0.7
0.3
that to which they were expected to have become
adapted. On, or shortly after, release they moved above
the upper limit of the free vertical range for the pre­
dicted depth of neutral buoyancy. Most remained in
midwater and several made repeated excursions to and
from the bottom for the remainder of the track. Five of
the 12 fish showed a slow progressive increase in depth
in midwater indicative of continuing gas secretion.
These observations suggest that the rates of gas secre­
tion in some of our caged cod were slower than those
observed in the laboratory experiments. This conclusion
is borne out by the rates of descent observed subse­
quently with fish in midwater. None was faster than the
predicted rate but several were slower and, although all
were within the 95 % confidence limits for the relation­
ship between the mean rate of adaptation and temper­
ature given by Harden Jones and Scholes (1985), some
(Table 1) were less than half the predicted rate. Cod 3,
for example, which was released at the surface, de­
scended at a rate of approximately 0.5 m h ' 1 - sub­
stantially slower than predicted - and had apparently
not reached equilibrium before the end of the track.
Cod 11, on the other hand, which was released from a
cage, came up to a depth of 10 m on release and sub­
sequently descended at a rate very close to the predicted
rate of 0.7 m h 1 (Fig. 4). Although several of these fish
went to the bottom for varying periods during the
course of the descent, none moved above the upper
limit of the free vertical range corresponding to the
mean rate of descent.
In addition to the rather slow progressive increase in
depth in midwater indicative of continuing gas secre­
tion, most cod exhibited rapid vertical movements both
on release and at the beginning and end of each major
midwater excursion. Most of the cage-adapted fish
came up into midwater immediately on release, some so
rapidly that it was impossible to follow their movements
with the sonar because they were too close to the ship
while the cage was being retrieved. Those fish whose
0-1
20-
E
c
■C
Q.
Q>
O
40-
60GMT
-|—
i—
i t t ~
t
—
i—
i
16
27
18
20
April
22
i—
i—
i—
i
24
i—
i—
i—
i—
i
02 04
28 April
06
r
08
i
10
n
12
i
i
i
14
i
16
i
r
18
Tide
Figure 4. The depth of cod 11 by day (sunrise 0430 h) and night (sunset 1910 h) in relation to the predicted depth of neutral
buoyancy on release (arrow). The mean rate of descent is indicated by the solid line and the equivalent upper and lower limits of
the free vertical range by dashed lines. Open bars indicate north-going (N) tides and black bars south-going (S) tides.
behaviour on release could be monitored ascended at
rates up to 3 m min“ 1 (Table 2) and experienced pres­
sure reductions of between 30 and 60 %. Vertical move­
ments of comparable rapidity occurred throughout
many of the subsequent tracks. Most occurred at rates
within the range 0.5—1 m m in-1, although two descents
were observed at speeds in excess of 3 m min-1.
D epth o f neutral buoyancy
A fish in neutral buoyancy in midwater becomes pro­
gressively more dense as it descends towards the bot­
tom, and its descent is assisted by its increasing negative
buoyancy. Rapid descents to the bottom, therefore,
reveal little about the level at which the fish is in neutral
buoyancy. Rapid ascents, however, can demonstrate
that the fish cannot have been in neutral buoyancy both
on the bottom and in midwater. The rates of movement
for rapid vertical ascents in excess of 10 m from the
bottom are shown in Figure 5 plotted as ln(P,/P2)
against time, where P, and P, are the pressures in atmo­
spheres corresponding to the greater and lesser depths,
respectively. The rates of ascent are outside the 95 %
confidence limits for the mean rate of gas resorption
determined experimentally by Harden Jones and
Scholes (1985), such that the fish cannot have been in
neutral buoyancy throughout the ascent. The ascents of
cod 11 between 0000 and 0015 h and 1230 and 1300 h on
28 April (Fig. 4) involved vertical movements of
26—35 m, rates of ascent of 2.3 and 0.9 m min ' and
pressure reductions of 57 and 45 % respectively. The
Table 2. Rates of ascent and percentage pressure reductions on release for fish not adapted to depth. P, and P, are the pressures
(ATA) corresponding to the initial and final depths (Arnold el al., subm.).
Fish
9
10
11
14
17
19
23
Time
interval
(GMT)
Initial
depth
(m)
Final
depth
(m)
Rate of
ascent
(m min-1)
Pressure
reduction
(% )
ln(P,/P2)
0630-0645
1249-1300
1519-1545
1046-1050
1155-1255
2230-2300
1450-1456
23
40
44
36
31
45
40
8
13
9
23
13
21
26
1.0
2.4
1.3
3.2
0.3
0.8
2.3
44
54
64
29
45
44
27
0.58
0.79
1.03
0.34
0.59
0.59
0.31
117
Figure 5. Time taken by acoustically
tagged cod to make vertical ascents in
excess of 10 m from the bottom in
relation to the magnitude of the
pressure reduction (ln(P|/P2)). The
iinear regression (Harden Jones and
Scholes, 1985) defines the time required
for a cod in neutral buoyancy on the
bottom to make a vertical ascent while
resorbing gas and remaining in neutral
buoyancy. The dashed lines indicate
9 5% confidence limits about the mean.
70
c
o
50 =
13
T3
a>
a>
i—
a
40 œ
CD
al
50
100
Time (minutes)
same argument applies to the vertical tracks of several
other fish, including the initial ascents on release shown
in Table 2.
Fish that are still adapting to depth are clearly neu­
trally buoyant in midwater and negatively buoyant on
the bottom. The same conclusion applies to cod 23
(Fig. 6), which was not adapted to the predicted depth,
and which showed no evidence of any further adapta-
20Figure 6. The depth of cod
23 in relation to the
predicted depth of neutral
buoyancy on release (solid
arrow, left). The dashed line
indicates the upper limit of
the equivalent free vertical
range. The dashed arrow,
right, indicates the estimated
depth of neutral buoyancy
calculated from the depth of
the shallowest vertical
excursion.
118
CL
a>
Q
GMT
40
60
20
21 April
Tide
24
04
22 April
08
24
04
23 April
E
c
JZ
Figure 7. The depth of cod
20 in relation to the
predicted depth of neutral
buoyancy on release. Details
as in Figure 6 and earlier
figures.
20-
CL
CD
O
40-
GMT
20 24 04 08
30 May 31 May
Tide
■
S
N
m
12
16
20
24
04 08
1 June
12
16
20
S
tion after its release from the cage. It made five ascents
involving pressure reductions of 32 to 51 % over periods
of 9 to 40 min. A similar argument applies to cod 20,
which on first inspection appeared to have been in neu­
tral buoyancy on the bottom (Fig. 7). During its mid­
water excursion between 1315 and 1345 h on 1 June,
cod 20 rose 13 m above the bottom, experiencing a
pressure reduction of approximately 30 %. The rate of
ascent shows that the fish could not have been in neutral
buoyancy on the bottom as well as in midwater. It seems
probable, therefore, that cod 20 remained in neutral
buoyancy at the original depth of 24 m - the depth of
the cage - and was thus negatively buoyant on the
bottom later in the track when the water became
deeper. Three fish only may have been in neutral buoy­
ancy on the bottom: cod 21. which remained on the
bottom at a depth of 28 m during 4-h surveillance after
150-h adaptation; cod 24, which made a single vertical
excursion of 6 m into midwater from a depth of 66 m,
before moving into shallower water; and cod 10, which
made limited excursions into midwater during the last
12 h of its track after some 30 hours of adaptation in
midwater.
P o te n t ia l
There are several areas of marine fisheries science in
which there are important behavioural problems am e­
nable to solution by the application of acoustic teleme­
try. Three areas are of obvious interest: gear and beha­
viour; behaviour in relation to acoustic survey; and
behaviour in relation to the environment. A substantial
amount of research is appropriate in all three.
G ear and behaviour
Rational controls on effort are needed in the heavily
exploited shellfisheries of England and Wales. In this
context, it would be useful to describe the behaviour of
lobsters and crabs in relation to baited pots and creels,
and to determine the effective radius of operation and
efficiency of the gear in relation to the mobility of the
animal. We have recently shown that it is possible to
track acoustically tagged lobsters (tagging can be ac­
complished underwater by a SCUBA diver) and think
that there is considerable research potential in this area.
Figure 8 shows the track of a lobster in Bridlington Bay
on the northeast coast of England in June 1986. This
animal was caught in a trap, tagged on the research
vessel, and released on the seabed by diver. It was
tracked by the research vessel for 62.5 h and showed a
clear pattern of diurnal activity, remaining stationary by
day and walking distances of several hundred metres by
night.
Similarly, there is scope for further studies of the
reactions of finfish to both fixed and moving gear in
relation to technological development and to rational
exploitation and conservation. Avoidance reactions
probably depend to a large extent on underwater visibil­
ity, and in this respect an acoustic technique permits
observations and measurements that would be difficult
to make either by diver or underwater television.
B ehaviour and acoustic survey
The problem of fish behaviour in relation to acoustic
survey remains essentially unresolved, and this is a po­
tentially fruitful research area. One question of partic­
ular concern is whether there are systematic changes in
target strength produced by behavioural or physiolog­
ical reactions to variations in the physical environment,
which could bias the results of acoustic surveys or sug­
gest modifications to the way in which they are carried
out.
Target strength is affected not only by swimbladder
volume, but also by changes in attitude on the part of
the fish, and tilt angles of a few degrees are sufficient to
introduce errors of up to 60 % into estimates of stock
abundance (Foote, 1980). Attitude may change system­
atically by day and night as a result of vertical migration
or changes in swimming speed (He and Wardle, 1986),
or as the result of an avoidance reaction to a research
vessel or its trawl. Changes in attitude could usefully be
studied in the open sea with the tilt tag recently devel­
oped at Lowestoft (Mitson and Holliday, 1990) for use
119
100m
0100
53'
2300
0600
2200
1500-1930
2200
0410
0300
0200
0800h
End
2100
2300
1936
2200
0200
2100
B rid lin gto n
0 1 0 0 , START
B ay
1904
0300-2130
2400
Figure 8. The track of an acoustically tagged lobster (Hom arus gammarus) released in Bridlington Bay on 14 June 1986 and
tracked for 62.5 h. Open circles indicate positions by day (sunrise 0330 h) and closed circles positions by night (sunset 2030 h);
positions were recorded at 30-min intervals, except where otherwise indicated. Position was initially determined by reference to
two long-life acoustic tags fixed on the seabed at locations 1 and 2, and later by reference to a m oored navigation buoy using radar.
The tag signal was lost between 2300 h on 15 June and 1500 h on 16 June, and again between 2230 h on 16 June and 0100 h on 17
June. The depth of water was 1 3 -1 7 m.
initially by the Norwegian Institute of Marine Research
in Bergen. Such studies could overcome some of the
limitations of existing experiments with caged fish.
B ehav io ur and the environm ent
Several stocks of fish on the European continental shelf
appear to be behaviourally rather than genetically iso­
lated. Because their migration circuits are related to the
movements of the water masses - to the residual circula­
tion during the egg and early larval stages, and to the
tidal streams for the adults - a simple model which
explained the distribution of populations in hydrody­
namic terms could be of practical use in fish-stock man­
agement. Some progress has been made at Lowestoft
towards the production of such a model (Arnold and
Cook, 1984), but there is scope for further work to test
its validity.
120
Two complementary approaches would appear to be
warranted. Further descriptive studies are required to
investigate the generality of the tidal stream transport
hypothesis. We need to know whether it is applicable
over the whole of the European shelf, or whether di­
rected movements (of the type we have observed in cod
and yellow eels in the coastal waters of the southern
North Sea) are more im portant in areas with slower
tidal streams. We also need to know whether it is appli­
cable only to demersal fish, or whether it could be
relevant to the pelagic species. Experimental studies are
needed to identify the factors that determine which
pattern of vertical migration is adopted and the length
of time for which it is maintained. We would also like to
know how the semi-diurnal pattern of vertical migration
is synchronized with the tidal streams to produce migra­
tion in a consistent direction and how the direction is
reversed between pre- and post-spawning migrations.
Technical d evelopm en ts
A knowledge of the scale and seasonality of fish pop­
ulation movements is basic to effective fisheries man­
agement. It is also fundamental to the assessment of
effects on the marine environment of dumping, marine
pollution, and the spread of disease. In all respects,
there is a need for a better understanding of the rela­
tionship between the behaviour of the fish and the phys­
ical. chemical, and biological features of the environ­
ment; and it is essential that these environmental factors
are measured at scales of space and time appropriate to
the processes under investigation (McCleave er al. .
1984).
Acoustic telemetry is particularly suited to the short
(hours and tens of metres) and - with a mobile system medium (days and tens of kilometres) scales, and has
the great merit that the movements of identified individ­
ual fish can be followed in the open sea independently
of the constraints imposed by confinement in cages or
experimental tanks. Measurements of well-chosen phys­
ical factors - for example, temperature, salinity, light
intensity, swimming speed - are particularly important
at these scales (Laurs e ta l., 1977; Westerberg, 1984)
and can be provided by the incorporation in the tag of a
sensor, which can measure the appropriate factor in the
immediate vicinity of the fish. Miniaturization of the
sensor is the principal constraint. At the longer scales
(months and hundreds of kilometres) difficulties arise
because of the dependence on a dedicated research
vessel and the short life - currently less than 100 hours of existing tags. The research vessel is expensive, may
not be available at the appropriate time, or for a suffi­
ciently long period, and may be prevented from work­
ing by adverse weather. Ways round these difficulties
are being sought, and are likely to include sonabuoys
and data-storage tags for the collection of data for peri­
ods of several weeks or even months on end. Sonabuoys
are appropriate to projects which require the collection
of data over a limited range for long periods. D ata­
storage tags, which would be retrieved through the fish­
ery, or which might ultimately transmit recorded data
by satellite (Hunter e ta l., 1986), would significantly
increase the rate of data acquisition, and would prob­
ably offer substantial cost benefits when compared with
existing techniques.
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